The present invention relates to phase locked loops which themselves have been employed in communication systems for many years. Phase locked loops have been employed in communication systems to generate, or regenerate, a "clean" replica of the carrier signal for demodulation purposes. Such loops are quite effective in tracking in that they follow small changes in the carrier signal with good fidelity.
In designing any phase locked loop a number of alternatives and tradeoffs are available depending on the particular application. For instance, increasing the bandwidth of the loop allows a wider capture range in that the loop is capable of locking up to signals whose frequency offset, from the natural frequency of the loop oscillator, is greater. The price which is paid for increasing the bandwidth of the loop is additional noise in the loop output.
In some communication systems a premium is placed upon rapid acquisition and loop lock. Typical characteristics of these systems include burst type communications such as voice operated communication links, TDMA (time division multiple access) and the like. In these systems the carrier signal is present only when information is being transmitted and to maximize the effectiveness of the system quick loop lock is desired for effective demodulation. One technique, known to the art, for decreasing acquisition time is to pre-set, by some manner or other, the loop oscillator to the expected carrier frequency. For instance, in a voice operated communication link the carrier is not present during the pauses which naturally occur in the speech process. However, it can be safely assumed that the transmitter carrier frequency will not change greatly during the period of the pause. Therefore, if the loop oscillator can be maintained at the frequency at which it was operating at the initiation of the pause it can be expected to be close to the actual carrier frequency at the termination of the pause.
The prior Klein et al. U.S. Pat. No. 3,885,238 is an example of a phase locked loop with pre-set capability although in a slightly different environment. Klein et al. provides a memory device 10 which monitors the error voltage output of the loop filter during the tracking phase of operations. The memory device is enabled to provide an error signal to the loop oscillator when the loop is not tracking to the end that the loop oscillator may be preset to the expected carrier frequency when the loop begins tracking. However, the Klein et al. arrangement has a number of drawbacks which the present invention seeks to overcome.
For example, when the Klein et al. loop is not tracking, the low pass filter 6 may still continue to produce an output voltage which, since the carrier is not present, will be generated by noise. Since this signal is coupled to the memory device 10 it may well contaminate the previously stored error voltage to the end that when the carrier again is present the loop oscillator may not be pre-set to the desired frequency. Furthermore, the loop filter exhibits capacitive impedence which implies that a time constant is associated with the filter. Since the memory device 10 provides the error voltage, when the loop is not tracking, to the loop oscillator, the loop filter is maintained at an essentially random voltage since its only input consist of a signal which is generated by noise. When the carrier signal again reappears the loop filter must either be charged or discharged to the correct error voltage prior to loop lock.
It is therefore one object of the present invention to provide an improved phase locked loop with pre-set and squelch features. More particularly, it is another object of the present invention to provide an improved phase locked loop which includes a track and hold circuit for storing therein a signal which is representative of a loop error voltage during the presence of the carrier. It is another object of the present invention to provide an improved phase locked loop where the track and hold circuit maintains a quantity representative of loop error voltage during the period when the carrier is absent and provides such signals to the input of the loop filter. It is a further object of the present invention to provide an improved phase locked loop which has squelch capabilities for disconnecting the input of the loop filter from the output of the phase detector to prevent noise from interferring in maintaining the loop oscillator pre-set at the desired frequency.